Create island mesh grid with drawn vertices?
There are several ways to make that profile with more or less even topology suitable for further landscape editing; one is to use curve's 2D type to fill the outline and then once converted to mesh edit the topology.

On the image below, from left to right:


original outline created as mesh;
outline converted to curve, its Z axis is clamped because it's set up as 2D curve but it's filled with faces;
outline converted to mesh and has 2 modifiers - Solidify and Remesh. The first one to give thickness to the outline as Remesh doesn't work with meshes without any thickness, the second one to generate new topology. Adding thickness is possible without modifier, e.g by extruding curve in the Geometry scroll.




Note that those tiny quads in the corners might not be desired as they can cause problems especially with any kind of subdivisions; it's best to change them before giving outline any thickness.